Aircraft: EMB ERJ 145 ER/LR
Phase of flight: Initial Approach
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence

We had planned to execute the ILS 27L with a flap setting of 22 degrees due to surface winds and reports of moderate icing below 7;000 FT MSL. Initially I was the pilot flying. We were vectored onto the 27L LOC outside of NRMAH; cleared to descend to 7;000 until GRABL and then execute the ILS 27L. We descended and leveled at 7;000 and began to accumulate ice. ATC asked us to slow (210 or 170; I don't remember which). As we started to slow I noticed an IAS flag and that my airspeed was rapidly dropping. I added power to correct the airspeed and crosschecked my airspeed indicator with the Captain's ASI. The Captain's ASI showed a slight increase in airspeed while my (First Officer) ASI settled around 60 KIAS. I alerted the Captain to the issue who took control of the aircraft and I assumed the duties of pilot not flying. The Captain coupled the autopilot to his side and rearmed the approach. The LOC immediately recaptured; but because we were below the glideslope the autopilot stayed in PIT mode with glideslope armed but not captured. However; I did not notice the issue with the autopilot mode because I was still mentally switching to pilot not flying duties; including communicating with ATC; and trying to analyze the issue. Just before crossing GRABL intersection (cleared to cross at 7;000 FT) I noticed that our altitude had drifted down to 6;700 FT due to being in PIT mode not ALT mode. I alerted the Captain; however it was too late to correct the altitude before crossing GRABL. Shortly after crossing GRABL we captured the glideslope. A couple of hundred feet after we captured the glideslope; ATC cancelled our approach clearance; told us to slow to 150 KTS and descend to 6;000 FT (from what I heard on the radio I believe this was due to separation with the preceding traffic). Before leveling at 6;000 FT ATC recleared us for the approach and repeated the instruction to slow to 150 KTS. As far as I'm aware; the only restriction we busted was the one over GRABL international. However; ATC changed our clearance several times in a very short period so it is possible that there was another mistake that I'm not aware of. After we were recleared for the approach and reestablished on both the LOC and glideslope I completed the Unreliable IAS QRH. We had completed the QRH and were fully configured and stabilized by the time we crossed the FAF. The Captain then completed an uneventful landing. Sometime during the approach I noticed that Pitot #2 was blocked by a disk of ice with an approximate diameter of 1.5 to 2 inches. The ice remained on the Pitot tube until landing. After arriving at the gate we called maintenance and wrote the aircraft up.As a side note; there was a slight delay in executing the QRH because I had a little trouble finding the appropriate checklist. At first I was looking in the Navigation/Flight Instruments Failure Index page. The checklist I was looking for was actually in the unannunciated error index pages. Unexpected mechanical issues; Unexpected change of flight controls and duties (pilot flying to pilot not flying) during high workload. Failure to verify autopilot modes after changing controls; Failure to crosscheck and monitor the aircraft; Slowness in changing duties (pilot flying to pilot not flying); Distracted by mechanical issues and QRH. Aircraft drifted below minimum altitude. Remember first and foremost to fly the airplane and not get distracted. EMB145 First Officer experiences an unannunciated airspeed indication failure at 7;000 FT during approach. Control is transferred to the Captain with the autopilot in pitch mode resulting in an altitude deviation prior to glideslope intercept. Visable ice is noted to have formed on the number two pitot tube.